COT (constant on-time) control method is widely used in power supply area due to its simple structure and excellent transient response.
FIG. 1 illustrates a schematic of a prior COT switching converter, wherein based on the input voltage Vin and output voltage Vout, an on-time control signal COT is generated by the on-time control circuit to determine the on-time of the high side transistor M1. The comparing circuit compares the output voltage Vout with a reference voltage Vref to provide a comparison signal SET. In accordance with the on-time control signal COT and the comparison signal SET, the logic circuit generates a switch control signal CTRL to control the high side transistor M1 and low side transistor M2. Generally, when the output voltage Vout decreases to be smaller than the reference voltage Vref, the transistor M1 turns on and the transistor M2 turns off. When the on-time of M1 reaches a time threshold determined by the on-time control circuit, the transistor M1 turns off and the transistor M2 turns on.
During mass production, the on-time control circuit is inevitably influenced by manufacture tolerance (e.g., tolerance on resistance, capacitance and comparator delay), which leads to different on-time for converters with same circuit design and work condition. Therefore, the product consistency is harmed.